The present invention relates to material processing using a beam of energy, such as a laser beam, an electron beam, a plasma arc or the like, and more particularly, to an apparatus and method for monitoring and controlling material removal from a workpiece by an energy beam, in real-time, during a material processing operation.
Material processing using a beam of energy, particularly a laser beam, is gaining wider application and use in manufacturing, such as the manufacturing of gas turbine engine components and the like, because of the speed, efficiency and ease of incorporation of a laser in an automated system.
Laser material processing may include a variety of processes performed on a workpiece, such as cutting, welding, drilling or the like, using a continuous wave or a pulsed laser beam. The average energy density of such a laser beam may range from as little as fractions of a joule/cm.sup.2 to as much as thousands of joules/cm.sup.2, the specific energy being dependent upon the particular process being performed and the type of material being processed.
Gas turbine engine components, particularly those used in high performance engines for aircraft propulsion, sometimes require the formation of extremely small diameter holes such as the cooling holes in a turbine blade. Holes of this type are sometimes referred to as "through holes" because the cooling holes extend completely through the surface of the turbine blade and form a channel to an interior plenum of the blade to permit cooling air streams to escape from the interior plenum and to flow over the exterior of the blade for blade cooling during engine operation. These cooling holes are typically formed by drilling using a pulsed laser beam. The laser beam pulse preferably has sufficient energy density to cause intense local heating of the workpiece when incident thereon. The intense local heating causes very hot minute particles of material to be expelled from the localized drilling area. The heated minute particles, in the form of a plume of sparks, radiate heat and light (optical emissions).
Current devices and methods for determining the amount of material removed during a laser material processing operation include checking the hole diameters with pin gauges after the material processing operation or performing air flow tests also after the operation to determine if a sufficient volume of air is passing through the blade cooling holes to provide adequate cooling of the component during engine operation. Both of these procedures are performed after the material processing operation and do not provide real-time monitoring of the quantity of material removed while the material processing operation is being performed to permit in-process adjustment of process parameters to improve efficiency or to correct processing errors. Additionally, the airflow procedure is not applicable for determining the amount of material removed from cavities or non-through holes, and depending upon the number of holes to be formed, it may not be practical or possible to pin gauge every through or non-through hole. As a practical matter, only a random sample of holes are generally pin gauged.
The above-referenced patents disclose devices and methods for detecting when the laser beam has completely drilled through the material to form a through hole so that the laser beam pulses can be stopped before damage may result to the interior of the component. The devices disclosed in these patents provide no information as to the quantity of material removed by each laser beam pulse or the cumulative amount of material removed by a series of pulses. The devices disclosed in these patents also do not provide analysis of the spark plumes or optical emission pulses for real-time in process monitoring of laser health or efficiency of the material processing operation, such as how well the laser beam is coupling into the material and removing material from the workpiece. Additionally, the devices disclosed provide no indication of the size or depth of through and non-through holes, cavities or other features being formed in the surface of a workpiece by a laser beam during the processing.
Another device for use in laser material processing is disclosed in U.S. Pat. No. 4,865,683, entitled "Method and Apparatus for Laser Process Control" by Burns, issued Sep. 12, 1989. This patent discloses a method and apparatus for controlling the energy of a laser beam incident upon a workpiece using a photodetector for generating an electrical signal which is proportional to the light reflected from the workpiece. The energy of the laser beam incident upon the workpiece is increased from some predetermined low level until the photodetector senses an abrupt change in the reflected light. This change represents a changed state in the workpiece material (from amorphous silicon to crystalline silicon), and this energy level is used as a reference level for further laser processing. This device also does not provide any information as to the quantity of material removed from the workpiece during the material processing operation.